. >> jim, an iraq war veteran and a registered nurse, was volunteering in the race's medical tent. >> i was convinced there was going to be another because what they used to do overseas sometimes is one would go off and then they would wait a little while. people would go to the aid of whoever was injured and they would set off a second one just to maximize their damage. >> just 12 seconds later, another explosion. >> we've had an attack. >> it was a deep feeling that almost just penetrated you to the core. >> dr. natalie stavas was running the race with her father. >> i could immediately sense that something was terribly wrong and chaos broke out almost immediately. they tried to stop us and they were actually trying to barricade the rest of us back. i leapt over the barricade. police officers were yelling at me, stop, you must stop. and i kept going. finally, a police officer actually grabbed my arm and he said, ma'am, ma'am, you must stop. and i said, i'm a pediatric physician. i'm a pediatric physician. you have to let me go. you have to. >> dr. stavas sprinted towards the finish